1. Field of the Invention
The present invention relates generally to microplasma-assisted desorption and ionization. In particular, the invention relates to a microplasma device serving as an ion source for a mass spectrometer.
2. Description of Related Art
Mass spectrometry is an analytical technique that identifies the chemical composition of a compound or sample based on the mass-to-charge ratio of charged particles. The technique requires a portion of the sample to be chemically fragmented and the fragmented segments to be ionized into charged particles. These particles are then passed into any type of mass spectrometer, which will determine their mass-to-charge ratio.
Three of the most common categories of mass spectrometers are known as time-of-flight mass analyzers, quadrupole mass analyzers, and ion trap mass analyzers. In each case, ions produced from the sample by the ion source are introduced using a variety of ion optics to guide the charged particles into the analyzer.
In a time-of-flight analyzer, the collection of ions are first accelerated through a region of known electric potential change. This gives each particle with the same charge the same amount of kinetic energy. The collection of accelerated ions are then allowed to travel through a region of zero electric field, and the time of their arrival at a detector at the end of this region is recorded. Particles with the same kinetic energy but different masses will travel through the “drift” region at different speeds, and thus reach the detector at different times. By this method the mass-to-charge ratio can be determined for each particle sensed by the detector.
A quadrupole mass analyzer operates by accepting the collection of ions into a region of oscillating electric field. By varying the parameters of this electric field the region can be made stable for a range of different mass-to-charge ratios. The quadrupole mass analyzer determines the mass-to-charge ratios for a variety of charged particles by quickly scanning through these stability parameters, keeping track of how many particles for each mass-to-charge ratio scanned through are detected.
An ion trap mass analyzer operates in a similar manner, but is capable of producing a field that is capable of trapping a number of particles with a range of mass-to-charge particles. The trap can modify the range of mass-to-charge ratios which are trapped, and thus by narrowing the stability region of operation certain mass-to-charge ratio particles can be released from the trap one by one and allowed to reach a detector outside, and the mass-to-charge ratio information recorded by the system. Other types of ion traps are capable of detecting the mass-to-charge ratio of charged particles in the trap without releasing them. This is accomplished by measuring the oscillation frequency of such particles in the trap by detecting the electromagnetic fields they produce, and analyzing the resulting data.
The use of electron, ion, and laser beams as an ion source for mass spectrometry-based imaging of surface and tissues is well known. Two popular approaches currently used are matrix assisted laser desportion ionization (MALDI) and secondary ion mass spectrometry (SIMS). These techniques are limited to monitoring the desorbed ion yields under high vacuum conditions and have been used to image semiconductor surfaces, insulators, polymers, tissues, and histological samples. Most MALDI and laser desorption/ionization based mass spectrometry approaches, however, are not effective under ambient temperature and pressure conditions. Some approaches such as desorption electrospary ionization (DESI), direct analysis in real time (DART), and radiofrequency plasma assisted desorption ionization (PADI) have been successfully used under ambient conditions. The spatial resolution of these approaches, however, is limited to the mm scale due to limitations inherent in the technology, and their reliance upon detecting ion signals produced as a result of surface or above surface interactions.
Therefore, there remains a need for an ion source capable of operating under ambient conditions which can be used to analyze condensed-phase targets such as liquids and surfaces with improved spatial resolution. The embodiments of the invention described below meet this need.